Mode multiplexing optical coupling device

ABSTRACT

An efficient tapered optical fiber bundle along with the method of manufacturing is presented. The tapered fiber bundle is fully fused to an induced shape with no interstitial space between fibers. To minimize fiber deformation and hence the tapered bundle&#39;s loss, the individual fibers are minimally deformed by positioning them in a fixture with predetermined geometry prior to fusion. The bundle could be optionally reshaped after fusion. The tapered bundle could then be used in its original form as a star coupler, or it could be cleaved and coupled to a multimode fiber, a multi-clad fiber, a cladding-pumped fiber, or an optical system to form an optical device. The resulting optical device has improved efficiency and lower loss compared with prior art devices.

FIELD OF THE INVENTION

The present invention relates generally to optical devices. Moreparticularly, the present invention relates to mode multiplexing opticalcoupling devices, such as mode multiplexing combiners for use with fiberlasers, fiber pumped solid-state lasers, and optical amplifiers, as wellas to optical couplers and splitters based on mode managed opticalcoupling.

BACKGROUND OF THE INVENTION

Fused coupler technology, wherein optical fibers are bundled together,heated, and pulled lengthwise, is commonly used to produce couplers,combiners and splitters for optical communication systems, medicaldevices and other industrial applications. Generally, a combiner is apassive fiber optic coupler in which power from several input fibers iscombined into one output fiber. Conversely, a splitter divides lightfrom a single input fiber into two or more output fibers. The couplerrepresents the general case where inputs from one or more fibers aremixed and distributed among one or more output fibers.

Combiners in particular are seeing new applications for diode pumpedlasers, including fiber lasers and solid-state lasers, and diode pumpedoptical amplifiers. They are used to combine multimode optical pumppower from a multiple sources, such as multimode laser diodes, andtransfer the combined pump power into the inner cladding of a multicladfiber or into a multimode fiber. A multiclad fiber typically has a smallcore (that typically transmits a singlemode or a small number of modes)surrounded by an inner cladding layer of lower refractive index andsignificantly larger cross-section that transmits the multimode pumppower. An outer cladding of even lower refractive index causes themultimode pump power to be confined in the inner cladding by totalinternal reflection. The multiclad fiber is used to combine a singlemode(or multimode mode) signal in the core, along with multimode pump powerin the inner cladding, to a separate device which may be used foramplification. These mode multiplexing combiners are typically used withcladding-pumped fibers. Cladding-pumped fibers are a special case ofmulticlad fiber where the multimode light propagates within the core andinner cladding interacting with special dopants (such as rare-earthelements like Er) in the core that absorb the pump photons and radiatephotons at a different wavelength. Under suitable conditions, thespecial dopants in the core cause stimulated or spontaneous emission atthe different wavelength and can operate in the form of a fiber laser oroptical amplifier. Multiclad fibers containing special dopants for thepurpose of lasing or amplification are known as cladding-pumped fibers.

For any coupler, splitter, or combiner, it is desirable to maximize thethroughput of optical power from any input fiber, through the device,and through any output fiber. For convenience, the case of a combiner isfurther described, recognizing that the same principles apply equally tosplitters and couplers. The throughput depends on efficientlytransferring the total brightness from all the input fibers into asingle output fiber having sufficient capacity to carry the combinedbrightness. This transfer can be analysed using modal analysis,ray-tracing methods, or by simple matching of input and outputbrightness. Conservation of brightness is based on the LaGrangeInvariant of an optical system and is typically characterized by thequantity etendue, which is the product of the area of illumination timesthe extended solid angle. For a fixed level of optical power, increasedbrightness implies a decrease in etendue. For a step index multimodeoptical fiber, the etendue can be approximated by E=π²/4 NA² D². If sucha step index fiber is tapered, its etendue remains constant while theeffective numeric aperture (NA) increases as the diameter (D) decreases.In this analysis, the NA refers to the maximum angle of light enteringor exiting the optical fiber according to NA=sin(acceptance angle).

In order to efficiently transfer power between two optical elements (inthis case from an input fiber bundle into an output fiber), tworequirements must be satisfied. First, the etendue on the input sideshould be less than or equal to the etendue on the output side,otherwise the coupling efficiency will be limited by E_(out)/E_(in).Second, the areas must be matched at the junction.

Prior art combiners, such as that described in U.S. Pat. No. 5,864,644to DiGiovanni et al., rely on the tapering process to eliminateinterstitial voids between input fibers, and to develop a suitablecircular cross-section in the input fiber bundle. However, it is notpossible to solely rely on the tapering process to achieve therequirements for low loss combiners, especially when there is a range inthe number of input ports, or if the output fiber is non-circular.

It is, therefore, desirable to provide an improved optical couplingdevice that substantially eliminates interstitial spacing between inputfibers, while providing a good cross-sectional match between the inputfiber bundle and the output fiber independent of the number of fibersbundled together.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at leastone disadvantage of previous optical coupling devices, such as combinersand splitters. It is a particular object of the present invention toprovide an improved optical coupling device that has reduced insertionloss in a wide range of combinations of fiber types, sizes, shapes, andnumber of ports.

In a first aspect, the present invention provides a tapered opticalfiber bundle. The tapered optical fiber bundle consists of a pluralityof optical fibers formed into a fiber bundle with a minimized encirclingradius. The bundle is adiabatically tapered, and heavily-fused into aninduced compact shape with fibers minimally deformed and no interstitialspace between the optical fibers. The optical fibers can be multimode,singlemode, multiclad, or cladding-pumped, and one or more of the portsmay be terminated. Terminated ports are fibers that are included in thebundle only to form part of the geometric or optical structure, but aresubsequently cut-off outside the fused region using a technique tominimize reflection from the cut endface.

In a further aspect, the present invention provides an optical fiberdevice, such as an optical combiner, an optical splitter, an opticalcoupler, for use in an optical system such as a cladding-pumped fiberlaser, fiber pumped solid-state laser, or a cladding-pumped opticalamplifier. The optical fiber device consists of a tapered fiber bundlecoupled to a multimode, multiclad, or cladding-pumped optical fiber, asecond tapered fiber bundle, or a bulk optical device (such as asolid-state laser element). The coupling process maximizes the transferof power (and signal) from the inputs to the outputs by virtue of theoptimized brightness contained in the input bundle.

In yet another aspect, the present invention provides a method ofmanufacturing the tapered fiber bundle. First, a plurality of opticalfibers, decoated to remove any polymer or metallic coating layers in theregion where they will be heated, are positioned in a predeterminedconfiguration that will result forming a minimized encircling radius.The positioned fibers are then twisted and bundled under controlledtension to result in a fiber bundle with minimized encircling radius. Anadhesive can be used at both sides of the bundle to secure thepositioning. The bundle is then heated and pulled to heavily fuse thefibers, while adiabatically tapering the bundle, into an induced shapewith no interstitial space between minimally deformed fibers. Ifdesired, glass cladding can be fully or partially removed prior tofusing the bundle. Also one or more singlemode (or multimode) fibers canbe incorporated into the bundle at appropriate locations such that theirposition in the resulting tapered bundle corresponds to similarsinglemode (or multimode) cores in an output bundle or multiclad fiberor cladding-pumped fiber.

To form an optical fiber device according to the present invention, thefused and tapered bundle is cleaved at the tapered region. The cleavedbundle endface is optionally reshaped by fusion splicing it to an outputfiber of appropriate shape, then recleaving again. The splicing andre-cleaving can be repeated, if desired, until surface tension duringfusion splicing causes the cleaved end to approach the desiredcross-sectional geometry. The reshaped cleaved end is then coupled to asuitable optical element, such as a single optical fiber, a secondtapered fiber bundle, or a bulk optical device (such as a solid-statelaser element). The method of the present invention can includepre-tapering of the output fiber, post-tapering of the junction betweenthe tapered fiber bundle and an output fiber or bundle, and re-coatingthe junction with a coating material, such as polymer or metallicmaterial.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawing(s) will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 is a schematic representation of a mode multiplexing opticalcoupling device according to an embodiment of the present invention;

FIG. 2 shows an exemplary 7-1 tapered fiber bundle according to thepresent invention;

FIGS. 3a-3 c are cross-sections of three multimode fibers bundledaccording to the present invention;

FIGS. 4a and 4 b are cross-sections of ten multimode fibers bundledaccording to the present invention;

FIG. 5 shows exemplary fixtures to secure fiber bundle configurations,prior to fusing and tapering, for two to twenty fibers, respectively;and

FIGS. 6a to 6 f are cross-sections of tapered fiber bundles having two,three, four, eight, ten and sixteen fibers, manufactured according tothe method of the present invention.

DETAILED DESCRIPTION

Generally, the present invention provides a mode multiplexing opticalcoupling device, and a method of making such a device. The opticaldevice, and related fabrication process, have one or more input fibersthat are fused and tapered and can be coupled to one or more outputfibers, such that the optimum cross-sectional shape of the fused regiondepends on the specific details of the output fiber or fibers. For easeof description, the following discussion is limited to the case of amode multiplexing combiner having a single output fiber of circularcross-section. However, this is not intended to limit the generality ofthe present invention, and it is clearly within the contemplation of theinventors that the present invention can be used with multiple outputfibers, and/or with output fibers having non-circular cross-sections.

The object of a mode multiplexing combiner is to maximize the transferof power by maximizing the number of multimode fibers of specified NAand core diameter that can efficiently transfer power into a multimode,multiclad, or cladding-pumped output fiber of specified dimensions andNA. To efficiently transfer power from a multifiber input bundle to anoutput fiber, the input fibers are ideally bundled to minimize theaggregate etendue, without causing microbending loss in the opticalfibers. A coupling device 20 according to an embodiment of the presentinvention is illustrated schematically in FIG. 1. The coupling device 20consists of a number of fibers 30 formed into a tapered fiber bundle 22,and spliced to an output fiber 24. The absolute minimum aggregateetendue occurs if the input fibers: first have all cladding removed,second are fused as a bundle to eliminate all interstitial space betweenthem, and third the fused bundle is in the formed to a circularcross-section. Such a device is problematic to fabricate because thetransmission loss is high due to the substantial core deformation. Inaddition, the structure is weak due to the complete removal of thecladding layer, which reduces the strength of the glass and prevents theuse of a structural adhesive on the unclad fiber. Instead, a practical,optimized configuration and process is required to minimize aggregateetendue of an input fiber bundle. This is achieved by:

i) selecting an initial geometric configuration of the input fibers tomaximize packing efficiency, such that the fibers are encompassed by thesmallest possible circle before fusing. This minimizes the deformationof cores when the fiber bundle is tapered and fully fused. The optimumgeometric configuration is not always based on hexagonal close-packedgeometry.

ii) fully fusing the fibers to obtain a degree of fusion close to one,thereby eliminating interstitial space between and around the fibers.

iii) selectively removing some or all of the fiber cladding from some orall of the input fibers to further reduce the aggregate bundle etendue,allowing any remaining cladding to amalgamate, and allowing the cores ofthe fibers to deform by a minimal amount in order to obtain asubstantially circular bundle.

iv) tapering the fibers with the primary objectives of (a) maintainingadiabatic tapering conditions to avoid power loss in the bundle and (b)promoting radial fusion between the fibers.

Generally, in the case of polymer clad fiber, the cladding is alwaysremoved because it is not compatible with the fusing process. In thecase of glass clad fiber, the cladding can be partly removed (forexample, by chemical etching) when specifically required to reduce theetendue of an input bundle below that of the output fiber.

As used herein, the packing efficiency (PE) is the ratio between thearea of a circle circumscribing the fiber bundle before fusing (Ax) andthe sum of the area of all the untapered fibers in the bundle (A₀). Theobject is to optimize the initial fiber orientation to minimize PE. Thedegree of tapering (DOT) is the ratio of the tapered core diameter tothe untapered diameter. The degree of fusion (DOF) ranges from 0 beforefusion occurs, to 1 when the fibers are completely fused into a circularcross-section. DOF is based on the area of a circle circumscribing allof the fibers in the bundle (A).

DOF=(A _(x) −A)/(A _(x) −A ₀).

It is preferable to minimize etendue by obtaining a DOF of 1, whichrequires the fiber bundle to contain no interstitial spaces and to havea perfectly circular perimeter, after the fusing process and thesubsequent splicing process. The deformation ratio (DR) is the ratio ofthe largest width to the smallest width of any deformed core shapewithin the fused and tapered bundle. This value is 1.0 if the core isnot deformed, and generally low propagation losses are observed if DRremains below 2.0.

Once a fused bundle with DOF of 1 has been fabricated, the objective isto efficiently transfer power from the input fibers into an outputfiber, the cross-sectional areas of the input and output elements arematched as closely as possible, since the etendue can not be increasedor decreased by further tapering of either side. Efficient powertransfer can be enhanced by:

i) matching the fiber/bundle diameter on both sides, either bycontinuing to taper the input bundle side, or by pretapering the outputside;

ii) fusion splicing the two sides together;

iii) optionally recleaving and resplicing the two sides very close tothe previous splice, using the surface tension during the successivefusion splices to assist in matching the shapes between the two ends;

iv) optionally post tapering the region of the fusion spliced junction.

The etendue of a fiber bundle cannot be reduced by merely tapering thefibers to match the size of the output fiber—therefore limiting low lossdevices to those where the etendue of the input fiber bundle is lessthan or equal to that of the output fiber before fusing and tapering. Ithas been found that the preferred manner in which to reduce the etendueof the input fiber bundle is to remove interstitial space between theinput fibers and obtain a circular bundle perimeter, which requiresextensive fusing. In practice the pulling process used to taper theinput fiber bundle facilitates fusing, often leading to bundle taperdiameters smaller than the output fiber diameter. Therefore, apretapered output fiber 28, as illustrated, is used to minimize lossesby matching the diameter of the fully fused fiber bundle. An outputtaper ratio of final diameter to initial diameter of ˜0.6, and a taperlength of ˜3 mm are currently used. In a presently preferred embodiment,the tapered region 28 of the output fiber 24 has its polymer claddingremoved, exposing the tapered region 28 to air.

As an example, consider the case of three input multimode step indexfibers, each with 105 micron core diameter, 125 micron claddingdiameter, and 0.22 NA, as shown in FIGS. 3a-3 c. An unfused, untaperedbundle of these fibers will have an overall diameter of 269 microns, andan effective NA of 0.22, as shown in FIG. 3a. The circumscribed area isA=Ax=56,855 μm² for a degree of fusion of 0. The etendue of the bundleis 8,642 μm²-str. Tapering the bundle will not change the etendue.Removing the interstitial space to a DOF=1 without removing thecladding, as shown in FIG. 3b, results in a final diameter of 216microns with an effective NA of 0.22 for an etendue of 5,572 μm²-str, ora 36% improvement over the unfused bundle. By completely removing allthe cladding and again eliminating interstitial space to a DOF=1, asshown in FIG. 3c, results in a final bundle diameter of 182 microns andan effective NA of 0.22, for an etendue of 3,956 μm²-str. Thisrepresents an additional 18% improvement, and is the lowest possibleetendue for the such a fiber bundle.

In a second example, as shown in FIGS. 4a and 4 b, ten input fibers areformed into a bundle. There is more than one way to arrange the fibers.The base area of ten fibers with 125 micron diameter is 12,718 μm². FIG.4a shows the natural hexagonal closed packed configuration, which isoften assumed to be the optimum packing configuration. The area of thecircle around the fibers is 271,547 μm², resulting in a PE of 2.2. FIG.4b shows an 8 around 2 configuration. This configuration is contained bya smaller circular diameter before fusing, having an area of 187,805μm², for a PE of 1.5. While both configurations can fuse to a circularbundle with DOF=1, the configuration of FIG. 3b, because of its smallerPE, will more easily form a circular bundle and will result in a fusedbundle with lower deformation of the cores.

FIG. 2 shows a cross-sectional view at the splice 29 (as shown inFIG. 1) of an exemplary 7-1 input fiber bundle. The input fiber consistsof seven 125 micron multimode fibers. The input fiber bundle is taperedover the tapered region 22, such that at the splice 29 the fibers aretight-packed, and heavily fused. This results in the individual fibersundergoing induced deformation such that the cores 31 a and 31 b deformfrom their initial circular shape, and the cladding 32 surrounding thefibers is amalgamated and redistributed to form an overall circularshape. Typically, the input fiber bundle taper length over the taperedregion 22 will be ˜3 mm, and the ratio of the diameter of the cleavedend to the initial diameter of the bundle will be ˜0.25. This is shownin actual cross-sections of tapered fiber bundles in FIGS. 6a-6 f. FIGS.6a-6 f also demonstrate the absence of interstitial spacing between thefibers and near circular cross section achieved by the method of thepresent invention for two, three, four, eight, ten and sixteen fibers.

In addition to using multimode input fibers, one or more singlemodefibers can be incorporated into the bundle. For example, if it isdesired to couple multimode pump light into the inner cladding of amulticlad fiber while simultaneously coupling singlemode light out of orinto the singlemode core of the same multiclad fiber, the central fiber31 b an be a singlemode fiber. Such simultaneous singlemode andmultimode coupling to a multiclad or cladding-pumped fiber facilitatesthe transfer of signal and pump power to a cladding-pumped fiber for theconstruction of devices such as cladding-pumped fiber amplifiers.Similarly, a central fiber that couples multimode light, or only alimited number of modes into, or out of, a multiclad or cladding-pumpedfiber can be used.

A tapered fiber bundle according to the present invention can bemanufactured as follows. First, individual input fibers are arranged ina predetermined configuration and are decoated. The glass cladding ofthe input fibers can be left on the fibers and incorporated into thetapered fiber bundle, or, optionally, the cladding can be removed orpartly removed prior to tapering. Typical multimode fibers have a 100micron diameter pure silica core, surrounded by a 125 micron outerdiameter fluorine doped silica cladding, and a numerical aperture of0.22. A rotatable fixture, can be used to maintain the fibers in thepredetermined configuration. Exemplary configurations for forming inputfiber bundles with from two to twenty input fibers are shown in FIG. 5.The illustrated configurations have been found to provide the desiredtight packed input bundle, and to ensure minimal deformation of theinput fibers. It should be noted that the optimum initial configurationof fibers in a bundle does not necessarily imply “hexagonal closepacking” or “maximum number of adjacent fibers”, but is the orientationthat minimizes etendue by arranging the fibers in the smallest circle(assuming the output fiber is circular). These optimum configurationsare achieved by correct alignment of tension controlled fibers prior tofusing and tapering using the patterns in FIG. 5, leading to the fusedexemplary geometries shown in FIGS. 6a-6 f.

The bundled fibers are twisted which applies normal compressive forcesbetween the fibers under tension. The twisted bundle is typically bondedon both sides to form a mechanically stable structure. The fiber bundlecan, for example, be bonded with a suitable adhesive, away from the heatof the fusion process. The bonding prevents individual fibers fromshifting relative to each other in subsequent steps, and maintainsdesired geometrical and physical stability. During the twisting andbonding, a precisely controlled longitudinal tension is applied to eachinput fiber to maintain the desired positioning geometry. Tensioning isin the range of from 5-20 gm.

The twisted and bonded bundle is then heated and pulled to form aheavily fused, tight-packed, tapered fiber region where the individualfibers, including their cores, are deformed. Thus, interstitial spacesbetween adjacent fibers are reduced or eliminated and the bundle isclose to circular in cross section. The heavily fused fiber bundle isthen cleaved to expose a cleaved end. In a presently preferredembodiment, the fiber bundle is cleaved near the center of the fusedarea.

The diameter of the bundle is measured and the output fiber ispretapered to match the bundle size. The bundle and pretapered outputfiber are then fusion spliced. Optionally, the bundle is recleaved about100-300 microns from the splice. The two ends are respliced together tobetter form a circular cross-section. Surface tension causes the cleavedend of the input fiber bundle to approximate the shape and size of theoutput fiber. This splicing and cleaving can, optionally, be repeateduntil the end of the input fiber bundle more precisely approaches adesired circular cross-section.

Optical fiber devices, such as optical couplers, combiners, splitters,fiber lasers and optical amplifiers, can then be formed by fusionsplicing the cleaved end of the input fiber bundle to an output fiber,such as a multimode, multiclad, or cladding-pumped fiber. In the case ofa multiclad or cladding-pumped output fiber, light transfer efficiencybetween the core of singlemode fiber in the input bundle and thecorresponding singlemode core of the output fiber can be enhanced priorto fusion splicing by heating the fiber to cause diffusion between thecore and cladding, thereby expanding the mode field of the singlemodecore. This will help compensate for the expanded mode field caused bytapering the corresponding singlemode core in the bundle. Furthermore,when splicing a singlemode fiber (in a bundle) to the corresponding coreof a multiclad fiber, high precision core alignment is necessary. Thetapered fiber bundle of the present invention can also be spliced to anoutput tapered fiber bundle to form a coupler, or can be coupled to abulk optical device (such as a solid-state laser element) if desired. Asplitter can be formed by using a combiner in the reverse direction, orby using a coupler with all except one input terminated.

The resulting optical fiber device can be post tapered to improve modematching between the input fiber bundle and the output fiber. Thejunction area between the tapered fiber bundle and the coupled outputscan also be recoated with a suitable coating material, such as polymeror metallic coatings.

Optical fiber devices according to the present invention have numerousadvantages over conventional optical devices. Insertion losses arereduced and coupling efficiency improved due to the degree ofcross-sectional match between the input fiber bundle and output fiber,the degree of fusion of the bundle, and the precision of the splicebetween the input bundle and the output fiber. At high powertransmission, low loss is a critical feature as optical losses willgenerate heat in the device.

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the invention, which is definedsolely by the claims appended hereto.

What is claimed is:
 1. A tapered optical fiber bundle, comprising: aplurality of input fibers formed into a fiber bundle, the fiber bundlebeing adiabatically tapered, and heavily-fused into an inducedcross-sectional shape with minimally deformed cores and no interstitialspace between the input fibers, wherein prior to fusing the input fibersin the bundle are arranged such that the cross-sectional shape has anencircling radius smaller than can be obtained with hexagonal packing.2. The tapered optical fiber bundle of claim 1, wherein input fibers areany of multimode, single mode, multiclad and cladding pumped fibers. 3.An optical fiber device, comprising: a tapered fiber bundle having aplurality of input fibers, adiabatically tapered, and heavily-fused intoan induced compact shape with minimally deformed cores and nointerstitial space between the input fibers at a cleaved end, whereinprior to fusing the input fibers in the bundle are arranged such thatthe cross-sectional shape has an encircling radius smaller than can beobtained with hexagonal packing; and an output element coupled to thecleaved end.
 4. The optical fiber device of claim 3, wherein the outputelement is another tapered fiber bundle.
 5. The optical fiber device ofclaim 3, wherein the output element is a single optical fiber.
 6. Theoptical fiber device of claim 5, wherein the single optical fiber is amultimode optical fiber.
 7. The optical fiber device of claim 6, whereinat least one of the input fibers is terminated to reduce backreflections.
 8. The optical fiber device of claim 5, wherein the singleoptical fiber is a double clad fiber.
 9. The optical fiber device ofclaim 5, wherein the single optical fiber is pre-tapered.
 10. Theoptical fiber device of claim 3, wherein the output element is fusionspliced to the cleaved end.
 11. The optical fiber device of claim 9,wherein a spliced junction between the tapered fiber bundle and theoutput element is post-tapered.
 12. The use of the optical fiber deviceof claim 3 as any one of an optical combiner, an optical splitter, acladding-pumped fiber laser, and a cladding-pumped optical amplifier.13. A star coupler, comprising: a tapered fiber bundle formed in themidsection of a plurality of fibers, adiabatically tapered, andheavily-fused into an induced compact shape with minimally deformedcores and no interstitial space between the fibers, wherein prior tofusing the fibers in the bundle are arranged such that thecross-sectional shape has encircling radius smaller than can be obtainedwith hexagonal packing, such that the plurality of fibers form input andoutput leads on each side of the fused bundle.
 14. The star coupler ofclaim 13, wherein at least one of the input leads is terminated toreduce back reflections.